A 55-year-old man with acute myocardial infarction has elevated serum troponin. His lactate dehydrogenase (LDH) isoenzyme pattern shows LDH-1 > LDH-2 ('flipped pattern'). The subunit composition of the LDH-1 isoenzyme that predominates in cardiac muscle is:
- A 4 M subunits (MMMM)
- B 2 M + 2 H subunits (MMHH)
- C 4 H subunits (HHHH) ✓
- D 3 H + 1 M subunits (HHHM)
Correct answer: C. 4 H subunits (HHHH)
Explanation
LDH-1 (heart isoenzyme) is composed of 4 H (heart-type) subunits arranged as HHHH tetramer, making it predominant in cardiac and RBC tissue. The flipped LDH-1 > LDH-2 pattern in serum reflects release of this isoenzyme from infarcted myocardium. LDH-5 (liver/skeletal muscle) contains 4 M subunits; mixed isoenzymes (LDH-2 through LDH-4) have varying M and H combinations.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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